An example of a variable valve control system is described in DE-OS No. 30 14 005 in which the actuating mechanism has a lift camshaft driven by the crankshaft. The lift camshaft, through a two-armed pivotable valve lever and a second cam that can be phase-rotated with respect to the lift camshaft, affects an intake valve of an internal-combustion engine. In order to make the valve lift curve of the intake valve changeable as a function of the operating conditions of the internal-combustion engine, the bearing shaft of the valve lever can be shifted relative to the axes of the lift camshaft and of the second cam which has an eccentric. Also, the angular position of the second cam relative to the lift camshaft can be controlled as a function of the load and the speed of the internal-combustion engine. By means of these two adjusting devices, an improvement of the efficiency of the internal-combustion engine for a partial engine load is achieved by changing the valve lift and the valve opening time. The disadvantages of this type of valve control system, however, is its high expense.
It is an objective of the present invention to provide a variable valve control system that can be manufactured at low cost and that changes the valve lift curve such that charge changing losses caused by the throttling are avoided without requiring a change of other adjusting quantities of the internal-combustion engine.
When a control camshaft rotating at the same speed is applied to the valve lever in addition to the lift camshaft, with the control camshaft affecting the pivoting motion of the valve lever, the intake valve can follow the same opening curve, while the charge volume is reduced only by the premature closing of the intake valve for all lift adjustments. The overlapping angle from the discharge valve to the intake valve maintains the same size for all valve lift curves that are adjusted in this way. By shortening the time cross-section, the charge volume is reduced without the occurrence of throttling and charge changing losses. In addition, by closing the intake valve as a function of the speed, a rise of torque is possible for maximum load conditions.
A feature of a preferred embodiment of the present invention is the disposition of the valve lever on a crank that can be pivoted at a stationary bearing point of the cylinder head. In contrast to conventional valve control systems, where the valve lever is pivoted around a stationary bolt, the valve lever according to the invention has an additional degree of freedom because it can be pivoted around two axes at the same time.
A further feature of a preferred embodiment is a roller disposed on the crank that is affected by the rotating control camshaft. On its cam, the control camshaft has a circular segment, the angular course of which is identical to the angular course of the cam of the lift camshaft. When the relative positions of the lift camshaft and the control camshaft are adjusted such that over the whole angular course of the lift camshaft, the circular segment of the control camshaft is also applied to the valve lever, the valve lever will carry out a pivoting motion as if it were disposed at a stationary bolt in the conventional way.
When the control camshaft is now rotated with respect to the lift camshaft, the valve lever, when the lift cam is arrested, can travel in the direction of the base circle of the control cam so that the intake valve is not completely opened and is closed again prematurely. Because the opening time is reduced in this way, the charge volume supplied to the cylinder is reduced. In addition, in the lower partial-load range, a desirable increased charging motion takes place. Because of the unthrottled intake and the higher pressure in front of the intake valve, the exhaust residual share is reduced, thereby improving combustion and efficiency.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, embodiments constructed in accordance with the present invention.